The GW detectors coming online in the next decades have great potential to shed light on the seeds of massive BHs, by directly observing their mergers at high redshifts. The Laser Interferometer Space Antenna (LISA), scheduled to launch in early 2030s, will be able to observe binary black hole mergers (BBHs) from few hundreds to \(\sim 10^6 \,{\rm M_\odot }\)  at \(z>10\)  [1]}. On the other side of the GW spectrum, next-generation (or third- generation, 3G) ground-based GW detectors with improved designs are being actively planned and investigated. Among them, detectors with emphasis on low-frequency sensitivity, such as the Einstein Telescope (ET, [2]}) and the Cosmic Explorer (CE, [3]}), can detect BBHs up to a thousand solar masses in the early universe [4]}. The mass spectra covered by LISA and 3G detectors are complimentary to each other and will provide our main discovery window into the mergers of BHs in the next decades.
